On the relationship between oxygen availability, energy state of the adenosine phosphate pool and protein synthesis in Rhodotorula gracilis

When cells of Rhodotorula gracilis are grown in a liquid mineral-glucose medium oxygen availability becomes a factor severely limiting growth at O2 partial pressures lower than 3 mm Hg. The present work deals with the changes of the adenosine nucleotide levels following the transition from a condition of excess O. to one where O. pressure is low enough to reduce by ATP 50% the rate of growth. Under this condition no detectable decrease of the ADP+AMP ratio is observed during a period of 10 minutes after the shift from «high» to «low» 0,. In the following period a very slight decrease (ca. 10%) of ATP and some increase of ADP and AMP are observed, while the total amount of adenosine nucleotides begins slowly to increase. Parallel experiments in which leucine-1-CH incorporation has been determined show that a very marked protein synthesis inhibition is already observed almost immediately (4-6 minutes) after the shift to the «low oxygen» condition, that is when no decrease of ATP or increase of ADP and AMP levels are detectable. Determinations of the radioactivity in the acid soluble fraction indicate that during this period leucine uptake from the medium is not significantly affected. The present results are thus interpreted as indicating that the marked (50%) inhibition of protein synthesis and of growth observed under the condition of low O. pressure is mediated by some mechanism different than a decrease of the energy state of the adenosine phosphate pool